Chrysotile asbestos is a fibrous or asbestiform magnesium silicate mineral and is the only member of the serpentine sub-group of asbestos. The individual fibrils of chrysotile, measuring about 300 to 400 A in diameter, have a scroll-like structure consisting of alternating layers of silica and brucite. The hydroxyl groups of the outermost brucite layer represent the surface of the fibrils. In contrast, a silica layer represents the surface in amphiboles and this fundamental difference is the reason for most of the differences in the physical properties of the two materials.
After conventional mining and milling, chrysotile asbestos is available as fibres, up to about 5 mm in diameter, which are in the form of bundles of fibrils. These bundles may be sub-divided further by mechanical action, and the degree of sub-division of the fibres is generally expressed in terms of average surface area of the product. Chrysotile asbestos fibre produced by a conventional mill ranges in surface area between about 3000 and 5000 cm.sup.2 /g and is generally provided in pressure packed form with a density of about 25 to about 50 lb/ft.sup.3.
Chrysotile asbestos is used in the manufacture of asbestos-cement products, such as, pipes and sheets. The manufacturing process consists of filtering a slurry of asbestos and portland cement (or portland cement partially substituted with silica flour) through a filter drum rotating in a vat. The thin layer of asbestos-cement formed continuously on the drum is transferred first to an endless filtration felt that moves over suction boxes, and is subsequently wound onto a mandrel.
It is necessary for this procedure for the chrysotile asbestos to be opened prior to formation of the slurry thereof with the cement, since the fibres produced by the asbestos mill are too coarse for effective use in this procedure. Opening of chrysotile fibres consists of mechanical action to separate these fibres into more fibres of smaller diameter. During the opening of the fibres, shortening of the fibres to a limited extent is unavoidable. It is known that fibres of shorter length provide lesser strength characteristics than fibers of longer length.
Water is removed from the asbestos-cement slurry at two stages of the above procedure: (1) on the filter drum, and (2) at the suction boxes. The rate at which the water can be removed is one of the most important properties of the asbestos-cement slurry, because the rate of water removal determines the rate of production of the products. In general, when chrysotile asbestos is used as the sole asbestos fibre in the slurry, the filtration rates obtained are poor (i.e., about 6 to about 12 ml/sec.).
Where "filtration rates" are referred to in this specification, the values given are those determined at a weight ratio of cement to asbestos of 7:1 and ratio of water to total solids in the slurry of 11.5 ml/g.
It is common practice, therefore, to substitute up to 30% by weight of the chrysotile by one or more other naturally-occurring asbestos fibre from the amphibole sub-group of asbestos, namely, amosite and crocidolite (blue asbestos), which possess high filtration rates (i.e., about 19 to about 20 ml/sec.), in order to improve the overall filtration properties of the asbestos cement. This practice has drawbacks, however, since amosite is normally considered to contribute little to the flexural strength of asbestos-cement products while crocidolite has been associated with health problems and its use in industrial applications is either prohibited or subject to severe restrictions. Moreover, both materials are in short supply.
Attempts to improve the filtration properties of asbestos-cement slurries by the addition of surfactants, coagulants, etc. have met with limited success. Similarly, treatment of the chrysotile asbestos with sodium silicate has been only partially successful in improving the filtration properties.
In U.S. Pat. No. 2,616,801, there is described a heat treatment operation for the improvement of the filtration properties of chrysotile asbestos in which the fibres are heated for very short periods of time not exceeding 3 minutes by passage through a combustion flame. The heat treatment is described as being effected at a temperature of 600.degree. to 1200.degree. F. (315.degree. to 650.degree. C.), although the procedure utilized does not permit close temperature control. The heat treatment in this prior art process is indicated to be accompanied by a loss of water of crystallization from the fibres and a corresponding strength loss.
A paper by the authors of the patent entitled "Heat Treatment of Chrysotile Asbestos Fibres", Canadian Institute of Mining, Transactions, Vol. LVIII, 1955, pages 33 to 37, describes a direct correlation between improved filtration properties and water loss and indicates that improved filtration properties cannot be attained without water loss and consequent strength loss.
Strength losses from the fibres are detrimental to the use of the fibres in asbestos cement products and should be avoided, if possible. As already mentioned, some strength loss may occur on opening of the fibres to the surface area required for asbestos-cement uses, but this is tolerated in the absence of a viable alternative.